Polyethylene films are used in a wide range of products and applications that generally fall into the categories of packaging and non-packaging. Packaging applications include food packaging, such as in-store produce bags and other containers for food; non-food packaging applications such as those for supported structures such as gaylord boxes, or those for containing various materials such as mulch bags, and other applications using stretch and shrink wrap films. Non-packaging applications include trash bags, can liners, construction film, such as vapor barriers in walls, and consumer products such as diapers.
Physical properties important in polyethylene films include tear strength, impact strength, tensile strength, stiffness and transparency. Overall film strength is desirable so that the films may be employed in applications without risk of physical failure. Transparency is an important property since it is desirable to have the flexibility to inspect items within a container without actually opening it. An additional property includes blocking, which refers to the ability or ease with which contacting film surfaces can be pulled apart.
Polyethylene films have been produced using different types of polymers to meet the demands of particular applications. Low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) can be heat-sealed and have good barrier characteristics. In addition, LDPE particularly excels in applications demanding high clarity, ease of processing and high gloss. LLDPE is especially useful for applications requiring tensile and impact strength properties. However, blends of the two polyethylenes typically result in films with good optical properties but poor physical properties, particularly for thin films. Additionally, particular applications require appropriate blocking properties for use in the commercial setting. Ziegler-Natta-based LLDPE's can typically achieve clarity with the addition of LDPE, but at the expense of only modest strength levels, as evidenced, e.g., by dart impact measurements. Compositions containing metallocene-based LLDPE's blended with LDPE typically achieve high clarity levels, but at a severe loss of strength properties. It remains an industry unmet need for polyethylene films that combine both high clarity and high strength properties with acceptable blocking.
Various attempts have been made to combine the favorable optical properties of LDPE and good structural properties of LLDPEs. U.S. Publication No. 2006/0047077 discloses plastic films prepared by coextruding a blend of 1.5 to 4.5 wt % of a high pressure low density polyethylene having a melt index of 0.1 to 0.8 dg/min, and 95.5 to 98.5% of a linear low density polyethylene. U.S. Pat. No. 6,130,293 discloses a composition containing 1 to 3 wt % of an ethylene homopolymer having a melt index of about 1 to about 4 dg/min and 97 to 99 wt % of a linear ethylene-α-olefin copolymer. U.S. Pat. No. 5,455,303 discloses films made from a blend of a reactor-made material containing a linear low density polyethylene and a C3-based material, with a low density polyethylene having a melt index of 0.2 to 20 dg/min. U.S. Pat. No. 6,870,010 discloses a low density, substantially linear polyethylene composition having a slice long chain branching index of 0.85 or less for any portion of the composition having a molecular weight of the 100,000 or above. U.S. Pat. No. 6,800,692 relates to polymer blends of metallocene-produced very low density polyethylene (VLDPE) having a density less than 0.916 g/cm3 and LDPE.
Copending U.S. application Ser. No. 12/655,427, published as U.S. Publication No. 2011/0160403, relates to blends of and LLDPE produced with a Ziegler-Natta catalyst and an LDPE having a balance of optical and physical properties.
Copending U.S. application Ser. No. 13/022,268 relates to a film comprising an LDPE having an MI of 0.1 to 0.6 dg/min and LLDPE produced with a single-site catalyst comprising a metallocene wherein the film has improved haze, dart impact, and Machine direction (MD)-Tear. U.S. application Ser. No. 13/022,268 does not disclose blocking properties for use in bag, bread bag, food bag or zipper bag applications.